The heart is the center of a person's circulatory system. It includes an electro-mechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the body organs and pump it to the lungs where the blood gets oxygenated. These pumping functions are resulted from contractions of the myocardium (cardiac muscles). In a normal heart, the sinoatrial (SA) node, the heart's natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to various regions of the heart to excite the myocardial tissues of these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various portions of the heart to contract in synchrony to result in efficient pumping functions.
Cardiac arrhythmia occurs when the heart's electrical activities become abnormal, such as when the SA node fails to generate electrical impulses at a normal rate, when the conduction of the electrical impulses in a portion of the electrical conduction system is blocked or abnormally delayed, when a pathological electrical conduction path form in the heart, and/or when a pathological focus generates electrical impulses to usurp control of the rhythm of the cardiac contractions from the SA node. A blocked or otherwise abnormal electrical conduction and/or deteriorated myocardial tissue cause dyssynchronous contraction of the heart, which may eventually lead to heart failure, a condition in which the heart fails to pump enough blood to meet the body's metabolic needs. Myocardial infarction (MI) is the necrosis of portions of the myocardial tissue resulted from cardiac ischemia, a condition in which the myocardium is deprived of adequate oxygen supply and metabolite removal due to an interruption in blood supply caused by an occlusion of a blood vessel such as a coronary artery. Following an MI, cardiac remodeling starts with expansion of the region of infarcted tissue and progresses to a chronic, global expansion in the size and change in the shape of the entire left ventricle. The consequences include a further impaired hemodynamic performance and a significantly increased risk of developing heart failure.
Cardiac pacing is applied to treat various cardiac disorders, such as cardiac arrhythmia, heart failure, MI, and cardiac remodeling, by delivering pacing pulses to the heart to restore the heart's normal rhythm and/or control timing of contraction on one or more regions of the heart. However, when the timing and/or intensity of cardiac pacing are inappropriate, cardiac arrhythmia and/or other cardiac disorders may be worsened. A patient may suffer from more than one of such cardiac disorders and may benefit from several types of cardiac pacing therapy modes. Also for safety, effectiveness, and other reasons, a cardiac pacing therapy may be delivered intermittently. Therefore, there is a need for control timing and/or intensity of cardiac pacing in a safe manner when the patient receives a cardiac pacing therapy that includes mode changes and/or intermittent delivery.